A firm latex foam mattress. It was inexpensive, it is great for my back, and the silly stuff with box springs sagging or breaking doesn't happen. (I had compared futons, including the *real* japanese style futons, and expensive posturological mattresses before settling on latex as the most sensible way to protect my back.) I'm impressed. Soft mattresses can sag and cause a lot of back problems.
Oh and a little known thing: if you're in an American city with the increasing scourge of bed bugs, I've heard that there are fewer places for bed bugs to hide (except for any mattress cover, bedding, frame, and other furniture). The latex foam is solid. With a traditional mattress/boxframe, you really have to toss them both.